The object of phosphating metals is to produce on the surface of the metals firmly intergrown metal phosphate coatings which, on their own, improve resistance to corrosion and, in combination with lacquers and other organic coatings, contribute towards significantly increasing paint adhesion and resistance to creepage on exposure to corrosive influences. Phosphating processes have been known for some time. Low-zinc phosphating processes are particularly suitable for pretreatment before painting. The phosphating solutions used in low-zinc phosphating have comparatively low contents of zinc ions, for example of 0.5 to 2 g/l. A key parameter in low-zinc phosphating baths is the ratio by weight of phosphate ions to zinc ions which is normally &gt;8 and may assume values of up to 30.
It has been found that phosphate coatings with distinctly improved corrosion-inhibiting and paint adhesion properties can be obtained by using other polyvalent cations in the zinc phosphating baths. For example, low-zinc processes with additions of, for example, 0.5 to 1.5 g/l of manganese ions and, for example, 0.3 to 2.0 g/l of nickel ions are widely used as so-called trication processes for preparing metal surfaces for painting, for example for the cathodic electrocoating of car bodies.
Unfortunately, the high content of nickel ions in the phosphating solutions of trication processes and the high content of nickel and nickel compounds in the phosphate coatings formed give rise to disadvantages insofar as nickel and nickel compounds are classified as critical from the point of view of pollution control and hygiene in the workplace. Accordingly, low-zinc phosphating processes which, without using nickel, lead to phosphate coatings comparable in quality with those obtained by nickel-containing processes have been described to an increasing extent in recent years. The accelerators nitrite and nitrate have also encountered increasing criticism on account of the possible formation of nitrous gases. In addition, it has been found that the phosphating of galvanized steel with nickel-free phosphating baths leads to inadequate protection against corrosion and to inadequate paint adhesion if the phosphating baths contain relatively large quantities (&gt;0.5 g/l) of nitrate.
For example, DE-A-39 20 296 describes a nickel-free phosphating process which uses magnesium ions in addition to zinc and manganese ions. In addition to 0.2 to 10 g/l of nitrate ions, the corresponding phosphating baths contain other oxidizing agents, selected from nitrite, chlorate or an organic oxidizing agent, acting as accelerators. EP-A-60 716 discloses low-zinc phosphating baths which contain zinc and manganese as essential cations and which may contain nickel as an optional constituent. The necessary accelerator is preferably selected from nitrite, m-nitrobenzene sulfonate or hydrogen peroxide. EP-A-228 151 also describes phosphating baths containing zinc and manganese as essential cations. The phosphating accelerator is selected from nitrite, nitrate, hydrogen peroxide, m-nitrobenzoate or p-nitrophenol.
German Patent Application P 43 41 041.2 describes a process for phosphating metal surfaces with aqueous acidic phosphating solutions containing zinc, manganese and phosphate ions and, as accelerator, m-nitrobenzene sulfonic acid or water-soluble salts thereof, in which the metal surfaces are contacted with a phosphating solution which is free from nickel, cobalt, copper, nitrite and oxo anions of halogens and which contains 0.3 to 2 g/l of Zn(II), 0.3 to 4 g/l of Mn(II), 5 to 40 g/l of phosphate ions, 0.2 to 2 g/l of m-nitrobenzene sulfonate and 0.2 to 2 g/l of nitrate ions. A similar process is described in DE-A-43 30 104, but uses 0.1 to 5 g of hydroxylamine instead of nitrobenzene sulfonate as accelerator.
Depending on the composition of the phosphating solution used, the method by which the phosphating solution is applied to the metal surfaces and/or other process parameters, the phosphate coating on the metal surfaces is not entirely compact. Instead, it is left with more or less large pores of which the surface area is of the order of 0.5 to 2% of the phosphated surface area and which have to be closed by so-called "after-passivation" to rule out potential points of attack for corrosive influences on the metal surfaces. In addition, after-passivation improves the adhesion of a paint subsequently applied.
It has been known for some time that solutions containing chromium salts can be used for this purpose. In particular, the corrosion resistance of the coatings produced by phosphating is significantly improved by after-treatment of the surfaces with solutions containing chromium(VI). The improvement in corrosion prevention results primarily from the fact that the phosphate deposited on the metal surface is partly converted into a metal(II)/chromium spinel.
A major disadvantage of using solutions containing chromium salts is that they are highly toxic. In addition, unwanted bubble formation is more likely to be observed during the subsequent application of paints or other coating materials.
For this reason, many other possibilities have been proposed for the after-passivation of phosphated metal surfaces, including for example the use of zirconium salts (NL-PS 71 16 498), cerium salts (EP-A-492 713), polymeric aluminum salts (WO 92/15724), oligo- or poly-phosphoric acid esters of inositol in conjunction with a water-soluble alkali metal or alkaline earth metal salt of these esters (DE-A-24 03 022) or even fluorides of various metals (DE-A-24 28 065).
An after-rinse solution containing Al, Zr and fluoride ions is known from EP-B-410 497. This solution may be regarded as a mixture of complex fluorides or even as a solution of aluminum hexafluorozirconate. The total quantity of these three ions is in the range from 0.1 to 2.0 g/l.
DE-A-21 00 497 relates to a process for the electrophoretic application of colors to iron-containing surfaces with a view to solving the problem of applying white or other light colors to the iron-containing surfaces without discoloration. This problem is solved by rinsing the surfaces--which may be phosphated beforehand--with copper-containing solutions. Copper concentrations of 0.1 to 10 g/l are proposed for this after-rinse solution. DE-A-34 00 339 also describes a copper-containing after-rinse solution for phosphated metal surfaces, copper contents of 0.01 to 10 g/l being established in the solution. The fact that these after-rinse solutions produce different results in conjunction with different phosphating processes was not taken into account.
Of the above-described processes for the after-rinsing of phosphate coatings--except for chromium-containing after-rinse solutions--only those which use solutions of complex fluorides of titanium and/or zirconium have been successful. In addition, organic reactive after-rinse solutions based on amine-substituted polyvinylphenols are used. In conjunction with a nickel-containing phosphating process, these chromium-free after-rinse solutions meet the stringent requirements which paint adhesion and corrosion prevention are expected to satisfy, for example, in the automotive industry. However, for environmental and works safety reasons, efforts are being made to introduce phosphating processes in which there is no need to use either nickel or chromium compounds in any of the treatment steps. Nickel-free phosphating processes in conjunction with a chromium-free after-rinse still do not reliably meet the paint adhesion and corrosion prevention requirements on all the bodywork materials used in the automotive industry. Accordingly, there is still a need for after-rinse solutions which, in conjunction with nickel- and nitrite-free phosphating and subsequent cathodic electrocoating, reliably meet the corrosion prevention and paint adhesion requirements for various substrate materials. The problem addressed by the present invention was to provide a corresponding process combination of a phosphating process optimized in terms of environmental and works safety and a particularly suitable chromium-free after-rinse before cathodic electrocoating.